This invention relates generally to a control circuit for an AC motor and particularly to a control system for maintaining a two-phase reversible AC induction motor rotating selectively in either direction. While the invention may be employed with any two-phase reversible motor, it is especially attractive when used in conjunction with the motor-drive system in an adjustable motorized hospital bed and will be described in that environment.
A motor operated hospital bed usually includes a hand-held, push button-actuated control device for permitting the patient occupying the bed to remotely control various adjustments of the bed merely by selectively depressing different spring-loaded push buttons, which in turn vary the operation of a control system for the bed. Ordinarily, the mattress-supporting structure of the bed is articulated, being divided into four interconnected sections or panels, namely a back section, a center or seat section, an upper knee or thigh section, and a lower knee or foot section. One motor-driven adjustment that may be controlled by the patient raises or lowers the two knee sections where they join together, thereby controlling the position of the patient's knees. Another adjustment, under the patient's control, pivots or tilts the back section with respect to the center section so that the patient's back and head may be raised or lowered. A third motor-driven adjustment may be controlled by the hand-held push button-actuated control device to vertically adjust the entire mattress-supporting frame. Each of these three adjustments is achieved by a respective one of three reversible AC motors.
To safeguard the patient from hazardous electrical shocks from the voltages present in the motor drive circuitry, some non-electrical coupling (such as optical coupling) is preferably employed in the control system to electrically isolate the patient's hand control unit. For example, in U.S. Pat. No. 3,993,940, issued Nov. 23, 1976 in the name of Joseph A. Volk, Jr., and assigned to the present assignee, a control system, having optical or photo couplers, is disclosed for operating a two-phase reversible AC induction motor to vary the high-low adjustment of a hospital bed. In that prior control arrangement, a pair of solid state switches (preferably in the form of triacs) are selectively turned on to apply AC line voltage directly to a selected one of the two field windings of the motor to effect motor rotation in a selected direction to either raise or lower the bed. By energizing photo couplers in the gating circuits, AC voltage is used to selectively gate each triac, the gating voltage being in phase with that appearing across the main terminals T.sub.1, T.sub.2 of the gated triac. Hence, each time the T.sub.1 -T.sub.2 voltage completes a half cycle and goes through zero amplitude, the gating voltage is doing likewise. The gating voltage must then increase significantly before the triac is regated during the next half cycle. As a consequence, during an initial portion of each half cycle the triac does not conduct, and then when it does conduct electrical noise is produced, giving rise to radio frequency interference (or RFI) which may deleteriously affect the operation of medical electronic equipment in the immediate area.
In this prior control system, the shifted phase voltage, which has a magnitude about twice that of the line voltage, is applied to the inactive photo coupler which controls the non-conducting triac. This means that the photo couplers must have a relatively high voltage rating so that they do not break down under high voltage stress.
It is also necessary in prior control systems, like that shown in U.S. Pat. No. 3,993,940, to interlock the manually-operated switches so that it is impossible to render both triacs conductive at the same time, in which case there would be no motor rotation since in-phase voltages would be applied to both field windings.
The present invention overcomes these, as well as other, shortcomings of prior control systems for two-phase reversible motors. Moreover, the unique construction of the present control system renders it considerably less expensive than those developed heretofore.